People commonly begin Se’uda Shelishit by eating bread just prior to sundown toward the end of Shabbat, and they continue the meal past sundown until after nightfall. When Rosh Hodesh begins on Mosa’eh Shabbat, which happens occasionally, an interesting question arises concerning the recitation of Ya’aleh Ve’yabo in Birkat Ha’mazon after Se’uda Shelishit. As night has already fallen, Rosh Hodesh has technically begun, seemingly requiring the recitation of Ya’aleh Ve’yabo. On the other hand, the Birkat Ha’mazon is being recited over a Shabbat meal, which obviously was begun before the onset of Rosh Hodesh.
Four different opinions exist among the Halachic authorities concerning this issue. The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) rules that in such a case one follows the point when the meal was begun. Since one began the meal when it was Shabbat and before it was Rosh Hodesh, he recites Reseh and not Ya’aleh Ve’yabo. The Mishna Berura (Rav Yisrael Meir Kagan of Radin, 1839-1933) advances the precise opposite view, maintaining that since the Birkat Ha’mazon is recited after Rosh Hodesh has begun, one recites Ya’aleh Ve’yabo. And since he recites Ya’aleh Ve’yabo, reflecting the position that the determining factor is the point when Birkat Ha’mazon is recited, he cannot recite Reseh. The third view is that of the Taz (Rabbi David Halevi Segal, Poland, 1586-1667) and Rabbi Shneur Zalman of Liadi (1745-1812), who ruled that in this case one recites both Reseh and Ya’aleh Ve’yabo. Finally, the Kaf Ha’haim (Rav Yaakob Haim Sofer, Baghdad-Israel, 1870-1939) ruled that a person in this case recites neither Reseh nor Ya’aleh Ve’yabo.
The accepted Halacha follows the view of the Ben Ish Hai, which is also the ruling of Maran, that in this case one recites Reseh and not Ya’aleh Ve’yabo, as the determining factor is the point when the meal began. And thus although the meal extended into the night, one nevertheless recites Reseh and not Ya’aleh Ve’yabo.
Summary: When Rosh Hodesh begins on Mosa’eh Shabbat, even if one’s Se’uda Shelishit extended past nightfall, he includes Reseh in Birkat Ha’mazon and does not recite Ya’aleh Ve’yabo.
